As wireless communication has been becoming increasingly fast in recent years, bands used by signals to be sent are being widened and their dynamic ranges are being increased. In this situation, it is desirable for electric power amplifiers to have high linearity to minimize deterioration of signal quality. To make apparatuses compact, reduce operation costs, and solve environmental problems, there has been an increasing desire for electric power amplifiers that operate with high electric power conversion efficiency.
Ordinary electric power amplifiers have a trade-off between linearity and electric power conversion efficiency. To increase both linearity and electric power conversion efficiency, electric power amplifiers have been operated in a non-linear zone in which electric power conversion efficiency is high and distortion compensation, by which resulting non-linear distortion is removed, has been performed to maintain linearity.
When an output signal from an electric power amplifier is distorted, residual distortion is generated in a frequency band (stipulated by a lower frequency and an upper frequency) near the frequency of the output signal. Output signals from electric power amplifiers are expected to satisfy the Adjacent Channel Power Ratio (ACPR) standard, Spectrum Emission Mask (SEM) standard, and other standards. These standards are also referred to as spurious standards. That is, residual distortion is expected to satisfy spurious standards.
One of distortion compensation method is a pre-distortion method. In the pre-distortion method, distortion caused in an electric power amplifier is reduced by adding characteristics opposite to the distortion characteristics of the electric power amplifier to an input signal of the electric power amplifier in advance.
Japanese Laid-open Patent Publication No. 9-69733 is an example of related art.
Problems
It is known that an electric power amplifier that operates at high efficiency causes a memory effect. The memory effect is an effect in which an output corresponding to an input at a certain time is affected by an input at a previous time. In a type of distortion compensation based on a pre-distortion method, not only an input at the current time but also an input at a previous time are used to create a pre-distorted signal.
In this type of distortion compensation based on the pre-distortion method in which an input at a previous time is also used, an unbalanced frequency may be generated in residual distortion. This unbalanced frequency of residual distortion is expected to be suppressed. To restrict residual distortion, different standards may be used between a lower frequency band of the frequency band of an output signal and a higher frequency band of its frequency band.
An object of the technology in the present disclosure is to provide a distortion compensation apparatus that controls residual distortion in distortion compensation.